Variable-effect lighting systems are commonly used for advertising, decoration, and ornamental or festive displays. Such lighting systems frequently include a set of coloured lamps packaged in a common fixture, and a control system which controls the output intensity of each lamp in order to control the colour of light emanating from the fixture.
For instance, Kunins (U.S. Pat. No. 2,515,236) teaches a coloured light source comprising a fixture having a red lamp, a green lamp, and blue lamp, with each lamp being connected to separate output terminal of an autotransformer. The autotransformer is connected to an AC voltage source, and the core of the autotransformer is rotated by a motor so as to vary the voltage applied to each lamp and thereby control the colour of light emanating from the fixture. Although the light source taught by Kunins may be suitable for producing light of varying colour, the use of a motor and autotransformer is bulky and is not suitable for producing intricate colour displays.
More recently, multi-coloured light-emitting diodes (LEDs) have been used with electronic switches to improve the versatility of the lighting system. For instance, Kazar (U.S. Pat. No. 5,008,595) teaches a light display comprising strings of bicoloured LED packages connected in parallel across a common DC voltage source. Each bicoloured LED package comprises a pair of red and green LEDs, connected back-to-back, with the bicoloured LED packages in each string being connected in parallel to the voltage source through an H-bridge circuit. A control circuit, connected to the H-bridge circuits, allows the red and green LEDS to conduct each alternate half cycle, with the conduction angle each half cycle being determined according to a modulating input source coupled to the control circuit. As a result, the bicolour LEDS can be forced to illuminate continuously, or to flash. Further, the colour of light produced by each bicolour LED can be continuously varied between two extremes.
Although the light display taught by Kazar offers an improvement over prior variable-effect lighting systems, the control system and the H-bridge circuitry increases the complexity of the lighting system. Further, the rate of change of coloured light produced is restricted by the modulating input source. Therefore, the range of colour displays which can be produced by the light display is limited.
Phares (U.S. Pat. No. 5,420,482) teaches a controlled lighting system which allows a greater range of colour displays to be realized. The lighting system comprises a control system which transmits illumination data to a number of lighting modules. Each lighting module includes at least two lamps and a control unit connected to the lamps and responsive to the illumination data to individually vary the amount of light emitted from each lamp. However, the illumination data only controls the brightness of each lamp at any given instant. Therefore, the lighting system is not particularly well suited to easily producing intricate colour displays.
Murad (U.S. Pat. No. 4,317.071) teaches a computerized illumination system for producing a continuous variation in output colour. The illumination system comprises a number of different coloured lamps, a low frequency clock, and a control circuit connected to the low frequency clock and to each coloured lamp for varying the intensity of light produced by each lamp. However, the rate of change of lamp intensity is dictated by the frequency of the low frequency clock, and the range of colour displays is limited.
Accordingly, there remains a need for a relatively simple variable-effect lighting system which allows for greater variation in the range of colour displays which can be realized.